The current emphasis of this project is the identification and characterization of endogenous sequences which recombine with ecotropic murine leukemia viruses (MuLV's). The resulting recombinant viruses, termed mink cell focus-forming viruses (MCF's) are thought to be involved in leukemia induction by MuLV's. The initial studies indicated that an erythroleukemia virus and a lymphatic leukemia virus selectively recombined with different sets of endogenous proviral sequences to give rise to MCF's in NFS mice. Further studies suggest that endogenous ecotropic viruses recombine with sequences distinct from either the erythroleukemia or lymphatic leukemia viruses. The oncogenicity of the MCF's derived from erythroleukemia and lymphatic leukemia virus was examined. MCF's derived from lymphatic leukemia viruses were found to be oncogenic in NFS and AKR mice whereas MCF's derived from the erythroleukemia virus did not induce disease in these strains. Detailed structural analyses of the viral genomes indicate that the MCF's derived from lymphatic leukemia viruses inevitably derive their p15(E) genes and terminal sequences from the ecotropic parent whereas recombinants with the erythroleukemia virus frequently derive their p15(E) gene from endogenous mouse sequences.